Photoelectric tube



' H. STEYSKAL PHOTOELECTRIC TUBE Jl me 11, 1940.

Filed Jan. 28, 1938 M I e e .0 0t m n t ft vw A hm W m e H H PatentedJune 11, 1940 i azoisss PATENT" orrics I 2,204,388 i PHO-TOEL CTRIUTUBEHerbertySteyska-LlBerlin, Germany, assignor to General Electric.Company, a corporation of NewYork 1 ApplicationJanuary2 8, 1938, SerialNo. 187,521

1 Ill Germany March 3, 1937 rea s. 01...2504166) t i The presentinvention'relates to photoelectric tubes and more particularly to ligh'tse'nsitive tubes-which-depend'on secondary electrons for theiroperation. 7 In the prior tubes ofthis generaltype, -the anode isusually interposed between the primary and secondary cathodes, and ismaintained at a potential more pos'itivewith respect to the primarycathode than the secondary emission cathode. anated by the primarycathode when exposed to light necessarily move in the general directionof the anode on their'way to the secondary cathode and in many cases,actually strike the anode. All of the electrons emittedby the primarycathode theref'ore'do not reach'the secondary cathode for the productionof secondary electrons and a part of the'amplification which the tubeshould have islost.

An object of the present invention is to pro vide a phototubeof thesecondary emission type and having an electrode structure in which theprimary electrons, in their travel toward the secondary emissionelectrode, do notstrike the anode. Another object is to provide a tubeof this character in which the secondary electrons, in their traveltoward the'anodetake a'difierent path from the primary electrons intheir move ment toward the secondary emissionelectrode. A further objectis to provide a phototube of the secondary emission type in whichthe'anode is interposed between theprimary and secondary cathodes, andthe anode field is controlled in such amanner as to cause the anode toattract the secondary electrons butnot the primary electrons. A stillfurther object is to provide a phototube in which. the photoelectronsemitted by the cathode are electrically and physically constrained fromimpinging on the anode, and only i the secondaryelectrons emitted by'thesecondary emitting electrode are permitted-to reach the anode. Theseobjects are attained in brief by providing a shield meniber between theanode and theprimary cathode, the shield being charged to such apotential as to directthe primary electrons away from the anodebut stillpermit the secondary electrons emitted by the secondary cathodeto reachthe anode; The invention will be better understood in connection withthe following description and the accompanying draw ing in which'Fig. 1shows a sectional view" or" a phototube improved in accordancewith' thepresent invention and connected in arr-energizing circuit. Figs. 2 and-3 are perspective-views of modifiedformso-f the-improved tube; ligJrshows Consequently, the photoelectrons em- I a still further tubemodification; Figs. 5 and 6 are fragmentary sectional views taken alonglines 5-5, 65respectivelyin Fig. 1; Figs. 7 and 8arefragmentarysectional views taken alonglines ducting material, forexample copper, mounted in 7 position in'any suitable manner. Theinterior surface of the electrode 2 is coated with lightsensitivematerial such as caesiumoxide on silver,

as is shown inv Fig. 6, and applied in any suitable and well-knownmanner. This electrode when exposed to an external light source (notshown) serves the function of a primary cathode inproviding'photoelectrons. As shown in Fig. 5, the inner surface of theelectrode 3 may be coated in any suitable and well-known manner with amaterial such as barium oxide on nickel which profusely emits secondaryelectrons when struck by primary electrons traveling at a high velocity.The anode of the tube is composed of a metallic body 4 of essentiallypunctiform character which is supported at a position central of theenvelope.

=When the secondary cathode 3 is maintained at a high potential withrespect to the primary cathode 2 by means of a battery 5, and the anodeis maintained at a still higher potential with respect to the cathodeZQphotoelectrons produced at the inner surface of the electrode 2, whenlight impinges on this surface, are strongly attracted by the anode andthe secondary cathode. The

light may be directed through the opening 6 in the secondary cathode 3,or through the space between the electrodes 2, 3. Those electrons whichreach'the electrode 3 give rise to secondary prior art tubes, all of thephotoelectrons generated at the electrode Tdo not'reach the secondaryemitter electrodefi but instead; some of them go directlyto the anode 4,the amplification obtained is not an'optimum. This diversion ofelectrons iseliminatedorat least substantially reduced in accordancewith the present invention by providinga-shield member or patio 8rigidly supported in the envelope in any suitable manner While theshield member may take anysuitable shape,- it is preferred to give itand positioned-between the anode and the pri- 'mary cathode.

a configuration similar to that of the primary cathode. The shield maybe given any suitable potential, positive or negative with respect tothe primary cathode and for this purpose, may be connected to the commonsource of potential 5. If desired, the shield member may be connecteddirectly to the cathode 2. The electrical effect on the primaryelectrons introduced by the electrically charged shield member 8 isshown roughly by the dotted line a which indicates the path of a primaryelectron being diverted from the anode by the shield and moving towardthe secondary cathode 3. As stated hereinbefore, for every electronwhich strikes the secondary cathode, more than one secondary electronmay be generated, and the path of these electrons is indicated by thedotted line b which leads directly to the anode 4.

If the anode lead-in connection presents a conductive area comparable tothat of the anode proper, it is in some cases advantageous to shield italso. This may be done either by covering it with a layer of insulatingmaterial or by providing an extended portion of the shield 8 adequatefor the purpose stated.

Fig. 2 shows the application of the improved shields to an elongatedcylindrical form of phototube provided with two or more centrallypositioned anodes l0. Shields H of a hemispherical configuration may beprovided about each anode, as in the case of the single anode tube ofFig. 1. If desired, the anodes may be connected together within the tubeto form a loop, in which case a single shield may be employed havingsuch a shape as to be interposed between all parts of the loop and theprimary cathode. The primary cathode [2 may take the form of alight-sensitive coating of any suitable material such as caesium oxideon silver. and applied directly to a limited portion of the envelope.The secondary cathode I3 may be of a size comparable to that of thelight-sensitive coating l2 but positioned diametrically opposite fromthe latter. A foundation layer of metal may be applied directly to theglass, and secondary emission material such as barium oxide coated onthe foundation layer. This secondary emitter member preferably has anaperture l4 through which light may enter the tube and impinge on thelight-sensitive cathode 12. The light might also be directed through thespace between the coatings or even through the secondary cathode if itis sufficiently transparent. As in the case of Fig. l, the shields II,when charged to the proper potential with respect to the primary andsecondary cathodes, serve to redirect the photoelectrons away from theanodes and to cause them to strike the secondary emitter electrode. Acircuit arrangement similar to that shown in Fig. 1 may be employed toprovide the necessary electrostatic fields.

The tube shown in Fig. 3 is of the same general type as that illustratedin Fig. 1 except that the primary and secondary cathodes are applieddirectly to the envelope rather than being constituted of a coating onmetal members mounted apart from the envelope. The light-sensitivecoating I5 may be constituted of caesium oxide or any otherlight-sensitive material, and the secondary emitter coating I 6 may becomposed of barium oxide or any other suitable secondary emissionmaterial. Leads are taken from these coatings to the exterior of theenvelope through the reentrant stem [1. The anode may comprise a rigidrod I8 which terminates at the central position of the bulb. The shieldl9 may constitute a hemispherical member, as in the case of Figs. 1 and2, and connections from this member and all of the electrodes taken to asource of current such as a battery (not shown).

Fig. 4 represents a tube somewhat similar to that shown in Fig. 3 inthat the primary and secondary cathodes I5, iii are constituted ofcoatings applied directly to the envelope but in this case, the anodeand the electron shield or baflie are constituted of the well-knownheater and sputter shield members which would normally be provided intubes of this sort. In those tubes having a coating applied directly tothe envelope, it is customary, as shown in the Ruggles Patent No.1,568,694, to provide a heater for vaporizing the foundation metal andalso to provide a sputter shield for preventing the deposition of themetal on undesired portions of the envelope. In case two separatecoatings are employed, as shown in this figure, two heaters and twoshields would normally be provided for the purposes mentioned. After thecoatings have been applied, the heater and sputter shields couldconveniently be used for the regular anode, also as an improved shieldfor intercepting or diverting the primary electrons away from the anodein accordance with the present invention. However, it would be notedthat only the heater designated by the reference character 20 which isfurther removed from the cathode I5 is in the proper position to beshielded from primary electrons by the immediately adjacent shield l9and hence, only this heater can be employed as the anode. As in the caseof the previous figures, the primary and secondary cathodes l5, l6, alsothe anode 20 and the shield l9, may be connected to a common source ofcurrent which provides the necessary fields within the tube to cause thesecondary electron emission and shielding efiects.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A discharge device including a primary cathode of relativelyextensive surface area, a secondary emission electrode positioned inspaced relation to the primary cathode to receive electrons therefrom,an anode interposed between the primary cathode and the secondaryemission electrode and effective during operation of the device toreceive current from the secondary emission electrode, said anode beingessentially punctiform so as to offer little physical obstruction to thepassage of current from the primary cathode to the secondary electrode,and a baffle interposed between the anode and the primary cathode forshielding the anode from electrons emitted by the cathode.

2. A discharge device including a primary cathode in the form of apartial surface of revolution, a secondary emission electrode also inthe form of a partial surface of revolution and positioned in oppositerelation to the primary cathode to receive electrons therefrom, an anodeinterposed between the primary cathode and the secondary emissionelectrode and effective during operation of the device to receivecurrent from the secondary emission electrode, said anode beingessentially punctiform and being positioned approximately at the commoncenter of revolution of said cathode, and secondary emission electrode,and a baflie interposed between the anode and the primary cathode, saidbaffle interceptingsubstantially all straight lines between the anodeand the cathode, thereby to minimize collection by the anode ofelectrons said anode being essentially punctiform and being positionedapproximately at the common center of curvature of the cathode and thesecondary emission electrode, and a bafile interposed between the anodeand the light sensitive cathode for deflecting electrons emitted fromthe cathode and proceeding. toward the secondary emission electrodefsaidbafiie being of materially smaller area than the cathode.

- HERBERT STEYSKAL.

